1. Technical Field
This disclosure relates in general to multi-phase voltage regulators and in particular to optimized management techniques of the phase regulators in order to reduce power consumption, enhancing the energetic efficiency of the multi-phase power supply system.
2. Description of the Related Art
Multi-phase voltage regulators are widely employed for powering servers, CPUs, notebooks and workstations. Lately efforts have been made for optimizing efficiency of this class of regulators by dynamically intervening on the number of active phases depending on the level of current absorbed by the load. In fact, by tracing the efficiency graph in function of the load current and of the number of phases, it may be observed that at low loads an improved efficiency is achieved by decreasing the number of active phases. A technique called phase-shedding is used for switching off a certain number of phases when the electrical load decreases, in order to achieve an optimal efficiency curve.
A well known technical problem of phase-shedding is the latency (delay) in reactivating switched off phases in presence of a load transient.
To optimize the response of the multi-phase system, it is important that during a load increase transient a number of phases in excess of what would be necessary for coping with the new regime of augmented absorption be reactivated, among those available. In order to understand better the problem, it may be useful to make an example.
Referring to FIG. 1, let us suppose that a four phase system (1PH, 2PH, 3PH, 4PH,) is functioning with only one phase in order to improve efficiency given than the output current remains very low. If a high load transient (e.g. 100 A) occurs, it would be ideal to be able to immediately use all the four phases available for responding to the load transient in a decisive manner for minimizing a “hole” (sagging) of the output voltage, rather than having to wait that the switch-on threshold of the second phase be surpassed, as depicted in FIG. 2, and then again that of the third phase and so forth.
If this adaptive process would not occur and the system responded with a single phase for almost the whole transient, the output voltage would undergo an abrupt drop because a single phase could not sustain a large transient. In practice, the system would have to wait that the current delivered by the active first phase reaches the triggering threshold in order to switch on the second phase. When the current of the first phase summed to that delivered by the second phase reaches the triggering threshold level of the third phase, then also the third phase will be able to turn on and so forth. It is evident that the time for reaching a point at which all four phases are active becomes relatively long and in order to limit the drop of the output voltage to acceptable levels, large storing capacitors are used, thus increasing costs of the application.
The major part of commercially available multi-phase voltage regulators, use nonlinear response methods to load transients. Basically, a load transient is detected by setting a threshold on the output voltage or monitoring its derivative (re: the so called Load Transient Boost (LTB) technique, disclosed in US 2007/0236205-A1). When a load increase transient is detected, all available phases are switched on.
These methods respond violently (by turning on all the phases) and therefore it may happen that a transient of modest magnitude or relatively gradual be not detected or that a transient just surpassing the triggering threshold cause the switching on of all the phases (which in server applications may be up to six or even eight) when the switching on of just a reduced number of available phases would be sufficient.
Variable frequency control methods of voltage regulators, for example a constant-on-time VCOT system as that disclosed in U.S. Patent Application Publication No. 2013/0057240, by the same applicant, respond to load transients in a linear manner, by increasing the switching frequency in function of the increased current absorbed by the load.